The amount, in grams, of water, that the student originally started with would be 252 g
From the balanced equation of the reaction, the ratio of H2O to H2 is 1:1. Thus for every 1 mole of water that is separated, 1 mole of H2 is produced.
Recall that: mole = mass/molar mass
Mole of 28.0 g H2 produced = 28/2
= 14 moles
Equivalent mole of H2O would, therefore, be 14 moles.
Mass of 14 moles H2O = mole x molar mass
= 14 x 18
= 252 g
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